04BIS10110152013RecombinLect4

Lrodriguez 2013

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Unformatted text preview: 24 dc progeny) (1.2% ­ Actual results indicate 4 double crossovers (Pcv-v of 0.002) not 24. The reduced number of double crossover progeny, relative to the expected number, is called interference and it interference can be calculated as follows. can BIS101­001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez ©2013 BIS101­001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez 41 Calculating Interference Interference = 1- Coefficient of Coincidence, where Coefficient of Coincidence = observed double crossovers* expected double crossovers* ­ ­ Interference reveals that the probability of one crossover Interference event can influence (reduce or increase) the probability of the second crossover event. It is believed that interference is due to a regulatory process It that limits the number of exchanges that can occur in a cell or along a pair of homologous chromosomes. along * Can be calculated using %, frequency or number of dc BIS101­001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez ©2013 BIS101­001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez 42 Different Types of Interference Absolute Partial None Negative If dc=0 dc < expected dc expected=dc observed dc > expected 1 <1 0 - number Interference = 1- Coefficient of Coincidence, where Coefficient of Coincidence = % observed double crossovers % expected double crossovers BIS101­001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez ©2013 BIS101­001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez 43 Mapping Function ­ ­ ­ ­ ­ Recombination Frequency (RF) increases as the distance between two loci Recombination increases, up to a point. increases, RF of 1% is equivalent to 1 map unit (m.u.) or 1 centiMorgan (cM) Double and higher multiple crossovers cause underestimates of map distances Double based on RF. based Therefore, the best estimates of map distances are obtained from the sum of the Therefore, distances calculated from shorter subintervals (10 m.u.). distances However, a formula developed by JSB Haldane (1919) c...
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